Current automatic optical inspection (AOI) systems use standard illumination, optical imaging techniques and filters to differentiate the features being inspected to the surrounding bulk material.
When AOI is applied to the task of inspecting a laser microvia that is 50 microns in diameter and below, standard AOI systems fail to perform. Standard AOI equipment lack the resolution and capability to differentiate the conductor pad which is supposed to be exposed and free from any contaminants, when the top dielectric is removed as the laser beam drills down to the conductor pad.
The typical rejected microvia is one that has remaining thin film of dielectric on the surface of the conductor pad. This remaining thin film of dielectric can take the form of what is defined as smear, or left over material, either it be adhesive, polymer, glass or resin. If the AOI system fails to find partially removed dielectric at the bottom of a microvia, even the slightest amount of dielectric film or resin, the microvia when plated or filled with a conductor will short or electrically fail once the product is delivered to clients.
Standard AOI systems are limited in the viewing of microvias, since microvias typically have diameters down to 10 microns. This leaves few options for lighting and differentiating these thin films. In many cases the AOI systems do not even detect the thin film that can be a thin as 1 micron. In short the AOI systems have an unacceptable rate of false positives which leads to yield issues once more value is added to the multilayered printed circuit board or flex circuit. In the case of high density chip packaging, one failure could cause serious failures in the field if the microvia is partially drilled. In many cases next generation chip packaging will have nearly 40,000 microvias per chip package, only a few faulty microvias can ruin a product.